QuickBooks Online only allows one connection per user ID. If you create one instance using Cloud Elements, and a second instance with the same user ID, then the second instance will revoke access to the first one.

Quickbooks online has a sync token attached to each ID. When you perform a GET /invoices call, the ID will appear as: 1234|3. This translates to <id>|<sync token>

The sync token helps manage concurrent requests. Only the request with the most recent sync token will succeed. When you PATCH /invoices, send the ID with the most recent sync token.